In conventional continuous cooking has a pre-treatment arrangement with a chip bin been used, in which a first heating of the chips by low pressure steam to a temperature of 70-100° C. is carried out. A steam-treatment in a steam vessel follows the pre-treatment in which the chips are intensely heated with flash steam and/or live low pressure steam to 110-120° C. The thoroughly steamed chips are then slurried in a chip chute before being fed to the cooking process. This process requires large quantities of steam as well a number of expensive treatment vessels adding cost and complexity into the cooking system.
The extensive steam treatment and its implementation in several treatment vessels has been considered to be totally necessary in order to be able ensure that air and water bound to the chips are expelled, such that the impregnation fluid can fully penetrate the chips and such that air is not drawn into the digestion process with the chips.
Attempts have been made to integrate the chip bin with the impregnation vessel such that a simple system is in this way obtained.
Metso Fiber Karlstad AB's U.S. Pat. No. 3,532,594 shows a combined vessel in which steam treatment and the formation of a slurry take place in a single pressure vessel that is maintained at an excess pressure of 1-2 atmospheres. The system was used in a pulp plant in Sweden as early as the 1970s. In this case, an impregnation fluid is recirculated during the addition of black liquor that maintains the suggested temperature of 105° C. in a circulation that consists of withdrawal strainer (35)—pump (23)—heat exchanger (25)—outlet/central pipe (27). Steam flashed off from black liquor in a flash tank was also added in an additional central pipe together with an optional addition of fresh steam. The idea in this case was that all water vapour would be expelled through the superior bed of chips by steam, and that this water vapour could be withdrawn (ventilated) through the outlet 12. A powerful heat exchanger (25) was required in this system. There is a serious risk of malodorous non-condensable gases (NCGs) leaking out, via the inlet 13. It is also specified in this patent that it would be possible to remove totally the addition of steam and have only a reinforced indirect heating of the chips with the aid of a heating flow during the addition of black liquor. It is difficult to implement this heating technology since it requires very large recirculation flows and a large heating power in the heat exchanger in order to be able to heat the cold chips.
U.S. Pat. No. 5,635,025 shows a system in which chips are fed without a preceding steam treatment into a vessel in the form of a combined chip bin, impregnation vessel and chip chute. Steam treatment of the chips that lie above the fluid level takes place at this location by the addition of steam from a “steam source”, as does a simple addition of impregnation fluid in the lower part of the vessel.
U.S. Pat. No. 6,280,567 shows a further such system in which the chips are fed without preceding steam treatment into an impregnation vessel at atmospheric pressure where the chips are heated by the addition of hot black liquor that maintains a temperature of approximately 130-140° C. The hot black liquor is added just under the fluid level via pipes in the wall of the impregnation vessel and excess liquid is only drained from the slurry in an external steaming vessel.
SE 523850 shows an alternative system in which hot, pressurised black liquor taken directly from the digester at a temperature of 125-140° C. is added to the upper part of the steam-treatment vessel, above the fluid level but under the level of chips, whereby the black liquor whose pressure has been relieved releases large quantities of steam for the steam treatment of the chips that lie above the fluid level established in the vessel. Excess fluid, the black liquor, can in this case be withdrawn from the lower part of the vessel.
Thus, prior art technology has in most cases used steam treatment as a significant part of the heating of the chips, where the steam that is used is either constituted by fresh steam or by steam that has been obtained following pressure reduction of black liquor from the cooking step, the latter containing large amount of sulphur laden NCG gases. This ensures a relatively large flow of steam, with the associated consumption of energy, and it requires a steam-treatment system that can be controlled.
The steam treatment has also involved the generation of large quantities of malodorous gases, i.e. NCG gases, with a high risk of explosion at certain concentrations.
U.S. Pat. No. 7,381,302 (or U.S. Pat. No. 7,615,134) shows an arrangement in an attempt to avoid excessive volumes of steam flowing trough the chip bed. Impregnation fluids (BL1/BL2/BL3) are in this case added with increasing temperatures at different positions (P1, P2, P3) where the local pressure may be above the boiling point of the added liquor. Most of the volatile compounds in the black liquor added are bound to the withdrawn impregnation fluid (REC).
In SE 530725 (=US2009139671) is a further improvement of atmospheric impregnation vessels using hot black liquor shown. Here are knock down showers installed above the chip level in order to prevent blow trough of malodorous NCG gases.
It has surprisingly become apparent that the use of an atmospheric impregnation vessel, using hot alkaline black liquor for the major part of the steaming effect of chips, releases large quantities of wood acidity in the chips. In recent tests in impregnation of chips has as much as 1.5 m3/BDt wood of acidified liquid with no or neglectable residual alkali been withdrawn from early screen sections in the impregnation vessel. This large volume of acidified liquid with low residual alkali was found to have a distinct reddish terracotta colour quite different in colour than regular spent black liquor from alkaline cooks, as well as having a sticky malodorous scent. There are a number of possible cures for this situation, but most of them results in increased alkali losses in the withdrawn spent impregnation liquid. A problem associated with the low residual alkali level in the withdrawn spent impregnation liquid is that the chips close to the wall of the impregnation vessel, close to withdrawal position, are impregnated with the same liquid as last withdrawn from vessel. The chips close to the wall are thus not impregnated at requested alkaline conditions as the chips close to centre of impregnation vessel, which results in uneven impregnation conditions over the cross section of the vessel.
Another problem is that the released wood acidity brings about a dissolution of metal from the wood material due to acidic conditions, which content of metals is cumbersome for the subsequent process. Especially calcium has a tendency to form scaling in the equipment in form of calcium carbonate, said scaling activated by high temperature.
Yet another problem is that the large volumes of acidic fluid need a lot of alkali only for neutralization of the acidity. This creates further alkali consumption in the cooking process.
The principal aim of the present invention is to achieve an improved method and an improved system for the impregnation and heating of chips that have not been steam-treated, which method and system reduce the problems with formation of large volumes of acidic condensates during steaming.
A second aim is to reduce the amount of metals being brought into the cooking process early in the process, thus reducing the risk for scaling problems.
A third aim is to reduce the total alkali charge in the cooking process, such that a minimum amount of alkali is needed to neutralise the wood material after steaming.
A fourth aim is to establish even impregnation conditions at alkali conditions for the chips directly after steaming and preferably in the very same vessel as used for steaming.
The inventive method for the impregnation and steaming of chips during the manufacture of chemical pulp comprises following steps a to e;
Step a: chips are continuously fed without preceding steam treatment to the top of an impregnation vessel where impregnated chips are fed out from the bottom of the vessel. The chips are thus in the original state having its natural content of wood acidity.
Step b: hot impregnation fluid at a first temperature above the boiling point of the hot impregnation fluid is added to the impregnation vessel (3), via a pipe having the outlet end located below a chip level (CHLEV) established in the impregnation vessel and at a distance from the walls of the impregnation vessel, preferably in the centre, such that steam is released into the chip volume for steaming the chips. The impregnation fluid thus has a temperature above the boiling point at the prevailing pressure established in the impregnation vessel, which will generate steam during pressure release inside the impregnation vessel.
Step c: the impregnation fluid added establishes a fluid level in the impregnation vessel and where the chip level lies at least 1-2 meters, preferably 3-5 meters, over the fluid level and where the pressure at the top of the impregnation vessel that is essentially at the level of atmospheric pressure, ±0.5 bar(g) preferably ±0.2 bar(g). These are conditions that guarantee a low temperature in the vessel for the impregnation vessel and a steam release trough the pile of chips for steaming effect.
Step d: that a withdrawal of a first impregnation fluid for a first use takes place from the vessel at the level of the fluid level, from a first withdrawal volume located behind a first screen row mounted in the wall of the impregnation vessel. This withdrawal will extract most of the early steam condensate.
Step e: according to the inventive method is also an additional withdrawal of a second impregnation fluid taking place from the vessel at a level below the first screen row, from a second withdrawal volume located behind a second screen row mounted in the wall of the impregnation vessel, said level below the first screen row not exceeding the diameter of the impregnation vessel, and wherein the second impregnation fluid at least in part is returned into the centre of the impregnation vessel which is of different use than the first use of the first impregnation liquid. This method using two screen rows and recirculation of the last withdrawn liquid at least in part to centre of vessel will enable withdrawal of the large volumes of acidified treatment fluid from treatment vessel, containing dissolved metal ions and wood extractives such as to turpentine etc., thus avoiding need for alkali for neutralisation of these wood acidity, and the subsequent circulation could establish an even alkali profile over the cross section of the impregnation vessel for an even impregnation process of the wood chips.
According to a preferred embodiment of the inventive method is the first use for being part in the liquor flow sent to recovery, and not being part of fluid returned into the centre of the impregnation vessel. As this acidified waste flow contains less valuable content, as of cellulose or hemicelluloses, for the alkaline cooking process it is beneficial for an early extraction of this acidified liquid volume. It could be merged with other black liquor flows and sent to the recovery boiler, or merged with other acidic waste liquors from the bleach plant for further appropriate recovery of chemicals or fibre content.
According to another preferred embodiment of the inventive method is the amount of hot impregnation fluid fed in to the impregnation vessel in association with the fluid level exceeding 3 tonnes per tonne of wood and at a temperature of the impregnation fluid in the interval 115-170° C., such that the temperature of the fluid-wood mixture that is established at the fluid level is established within the interval 90-115° C., preferably within the interval 95-105° C., and where the level of alkali of the added impregnation fluid exceeds 15 g/l EA as NaOH. These amounts of hot alkaline liquor would supply all or most of the steam and alkali needed for the impregnation process.
According to yet another preferred embodiment of the inventive method is the amount of second impregnation fluid withdrawn exceeding 0.5 tonnes per tonne of wood and at least a part of this withdrawn second impregnation fluid is recirculated back to the centre of the impregnation vessel. Preferably could as much as 1.0 to 2.0 tonnes be withdrawn, and larger flows will establish a stronger circulation rate.
From this withdrawn amount could a part of the second impregnation fluid be returned into the centre of the impregnation vessel, and preferably at least 0.5 tonnes per tonne of wood. So if in total 0.5 tonnes is withdrawn could the entire volume be returned, and if in total 1.0 tonnes is withdrawn could half the volume be returned, hence always at least a part of the total amount is withdrawn and returned into the centre of the impregnation vessel.
According to yet a preferred embodiment of the inventive method could a part of the second impregnation fluid returned into the centre of the impregnation vessel be diluted with additional liquid. This additional liquid is preferably an alkaline wash filtrate from subsequent cooking or bleaching stages having a residual alkali content.
Further the total volume of second impregnation fluid, including any dilution, is preferably returned into the centre of the impregnation vessel below the first screens, and an upwardly directed displacement flow is established towards the first screens. The upwardly directed displacement flow will improve wash out effect of the acidic rest fluids from the preceding steaming effect, and further improve an even alkali profile in the subsequent impregnation process.
The inventive system used for impregnating and steaming chips in one single impregnation vessel during the manufacture of chemical pulp comprises following features. Said impregnation vessel having an inlet at the top for chips and an outlet in the bottom for impregnated chips. Said impregnation vessel having means for adding hot impregnation fluid at a first temperature above the boiling point of the hot impregnation fluid to the impregnation vessel, via a first central pipe having the outlet end located below a chip level established in the impregnation vessel and at a distance from the walls of the impregnation vessel, said outlet end preferably located in the centre, such that steam is released into the chip volume for steaming the chips. Said impregnation vessel further having means for establishing a fluid level by the added impregnation fluid in the impregnation vessel. And further having means for establishing a chip level lying at least 1-2 meters, preferably 3-5 meters, over the fluid level. The impregnation vessel further includes means for establishment of a pressure at the top of the impregnation vessel that is essentially at the level of atmospheric pressure, ±0.5 bar(g) preferably ±0.2 bar(g). Said impregnation vessel having a first screen row at the level of the fluid level comprising a first withdrawal volume located behind the first screen row mounted in the wall of the impregnation vessel for withdrawing spent impregnation fluid.
The inventive system further comprises an additional second screen row located in the impregnation vessel at a level below the first screen row, having a second withdrawal volume located behind the second screen row mounted in the wall of the impregnation vessel. Said level below the first screen row not exceeding the diameter of the impregnation vessel. And further a second withdrawal pipe connected to the second withdrawal volume for extracting spent treatment liquid, and wherein the second withdrawal pipe is connected to a second central pipe having an outlet in the impregnation vessel at a distance from the wall of the impregnation vessel and preferably in the centre of the impregnation vessel.
According to a preferred embodiment of the inventive system is the outlet of the second central pipe located below the first screen row. By having the first and second central pipes arranged at different levels could each central pipe be used to optimize either steam generation, as to the first central pipe, or improved circulation for evening out the alkali profile, as to the second central pipe. In some solutions however could only one single central pipe be used adding a mixed flow of hot impregnation fluid and recirculation fluid.
According to yet a preferred embodiment of the inventive system is a source of dilution liquid connected to the piping (43,41c) ending up into the outlet of the central pipe. By adding such source of dilution liquid could enhanced wash-out performance and a more even alkali profile be obtained.
The inventive withdrawal screen section for use in pre-treatment of chips in a liquor-vapour phase treatment vessel having a vapour phase in the top and a liquid phase in the bottom of said vessel comprises following components. Said withdrawal screen comprising, a first screen row mounted in the wall of the treatment vessel and in contact with chips drenched in treatment liquid inside the treatment vessel. Further a first withdrawal volume arranged outside of the first screen row collecting treatment liquid withdrawn from the treatment vessel via said first screen row as well as a first withdrawal pipe connected to the first withdrawal volume for extracting spent treatment liquid via a first pump. The inventive modification comprises further                an additional second screen row arranged at a level below the first screen row,        a second withdrawal volume arranged outside of the second screen row collecting treatment liquid withdrawn from the treatment vessel via said second screen row,        a second withdrawal pipe connected to the second withdrawal volume for extracting spent treatment liquid via a second pump (P2),        said level below the first screen row not exceeding the diameter of the impregnation vessel, and        wherein the second withdrawal pipe is connected to a second central pipe having an outlet in the impregnation vessel at a distance from the wall of the impregnation vessel and preferably in the centre of the impregnation vessel.        
In a preferred embodiment of the withdrawal screen is the outlet of the second central pipe located below the first screen row.